Fluorination catalyst and process

ABSTRACT

A chromia-based fluorination catalyst in which the chromia is at least partially crystalline and which may contain a zinc or a compound thereof, the production of the catalyst by sintering amorphous chromia and its use in fluorination processes.

This invention relates to a fluorination catalyst and the production anduse thereof and particularly to an improved fluorination catalyst basedon chromia, a process for producing the catalyst and a fluorinationprocess using the catalyst.

Fluorination processes comprising reaction of a starting material withhydrogen fluoride to introduce one or more fluorine atoms into thestarting material are well known and are used extensively in industry.Vapour phase processes in which the starting material and hydrogenfluoride are reacted in the vapour phase at elevated temperature arecommon and such processes usually employ a fluorination catalyst whichoften is a catalyst comprising or based on chromia which has beensubjected to a pretreatment with hydrogen fluoride to provide theworking catalyst. It is generally accepted that chromium oxide catalystsof high surface area and wherein the chromium is present as chromium(III) have high initial activity and that such active chromia catalystsare in an amorphous or essentially amorphous state. A recent developmentin chromia catalysts is a catalyst of enhanced activity produced byincorporating an activity-promoting amount of a divalent metal oxidesuch as an oxide of zinc, nickel or cobalt, especially zinc, in thecatalyst, the oxide or at least the chromia remaining in the essentiallyamorphous state and having a large surface area. Catalysts containingother divalent metal oxides such as magnesia have also been proposed.

When used in the production of hydrofluorocarbons [HFCs], the knownchromia catalysts and especially those promoted by a divalent metal suchas zinc have a high initial activity and can result in high conversionsand high selectivities. They suffer from a progressive reduction inactivity due to deposition of coke on the catalyst but they can beregenerated a number of times by heating in an oxygen-containingatmosphere such as air or a mixture of air with hydrogen fluoride andhave a reasonable and generally acceptable lifetime. However, thecatalysts suffer the disadvantage that they are not particularly robust,especially in respect of chemical robustness and are deteriorated underthe conditions of use and especially when subjected to high temperaturesin the presence of hydrogen fluoride so that their lifetime leavessomething to be desired.

The present invention is based on the discovery that the robustness ofchromia—based catalysts and hence their useful working lifetimes isincreased by inducing or introducing crystallinity and preferably acontrolled degree of crystallinity into the chromia. Moreover, theinitial activity of the catalysts can be slightly but significantlyenhanced, without a reduction in selectivity, by introducing anactivity-promoting amount of zinc or a compound of zinc into thecatalyst.

According to the first aspect of the invention there is provided animproved chromia-based fluorination catalyst wherein the chromia is atleast partially crystalline.

Preferably, the chromia exhibits an apparent degree of crystallinity asrepresented by alpha chromia type crystals greater than 8%, preferablygreater than 20%, and less than 50% by weight.

Introducing crystallinity into the chromia results in a decrease in thesurface area of the catalyst and too high a degree of crystallinityresults in an unacceptably low surface area, for example below 20 m²/gm.The degree of crystallinity in the catalyst can be controlled so as toresult in a catalyst having a surface area greater than about 20 m²/gm,preferably from about 30 to about 70 m²/gm.

According to a further aspect of the invention, there is provided animproved zinc-promoted chromia fluorination catalyst wherein the chromiais at least partially crystalline and the catalyst comprises zinc or acompound of zinc in an amount of less than about 3% by weight of thecatalyst.

In a further aspect of the invention there is provided an improvedzinc-promoted chromia-based fluorination catalyst wherein the chromia isat least partially crystalline produced by inducing crystallinity inchromia and subsequently introducing zinc or a compound of zinc into thecrystallised chromia by impregnation with a solution of a soluble zincsalt. The catalyst preferably contains from 0.1% to about 2% by weightof zinc or a compound of zinc depending upon the degree of crystallinityinduced in the chromia.

Inducing crystallinity in the chromia results in a decrease in thesurface area of the catalyst and a very high a degree of crystallinityresults in a very low surface area, for example below 10 m²/gm. Thedegree of crystallinity in the catalyst of the invention can becontrolled such that the catalyst has a surface area greater than about20 m²/gm, preferably from about 30 to about 70 m²/gm.

Suitably, the catalyst according to the first aspect of the inventioncontains zinc or a compound of zinc. A catalyst according to theinvention may contain an activity-promoting amount of a divalent metalsuch as cobalt, magnesium or nickel or a compound thereof in addition toor instead of zinc or a zinc compound. Nevertheless, the preferred metalis zinc and in this case the amount of the zinc is important since it isknown that zinc can act as a catalyst poison if present in too large anamount. We have found that whilst the activity-promoting amount of zincin catalysts wherein the chromia is amorphous is generally greater thanabout 2% by weight and usually greater than about 5% by weight dependingupon the method of production of the catalyst, the activity promotingamount of zinc in the partially crystallised catalysts of the inventionshould generally be less than about 2% by weight, preferably no greaterthan about 1% by weight.

According to a preferred embodiment of the invention there is provided achromium-based fluorination catalyst comprising from 0.1 to 2% by weightof zinc or a compound of zinc wherein the chromia is at least partiallycrystalline. The catalyst preferably has an apparent degree ofcrystallinity as represented by alpha chromia type crystals of fromabout 8% to about 50% and has a surface area greater than about 20m²/gm.

If present, the amount of divalent metal other than zinc in thecatalyst, whether the divalent metal be an activity promotor or not, isnot critical since such metals are not generally regarded as catalystpoisons even if present in large amounts. The amount of such metals mayvary over a wide range up to 50% by weight or even higher of thecatalyst, although the amount will usually be in the range from about 5%to about 25% by weight.

The apparent degree of crystallinity or the degree of crystallinityinduced in the chromia is determined by X-ray diffraction analysis usingthe standard NIST [National Institute of Standards and Technology]technique and comparing the result with that obtained by analysis of apure alpha chromia standard prepared by sintering chromia at 1223 k inair for 24 hours (100% crystallinity). The catalysts do not have a truealpha chromia structure so that the % degree of crystallinity determinedby comparison with the results for pure alpha chromia is not a true %degree of crystallinity and therefor is referred to herein as the“apparent degree of crystallinity”. Morover, since the catalyststructure is not true alpha chromia so that the X-ray diffraction peaktends to be slightly distorted, the apparent degree of crystallinity isexpressed herein as being represented by “alpha chromia type crystals”.

The apparent degree of crystallinity as represented by alpha chromiatype crystals is determined by measuring the integrated area of the 104peak of both the catalyst sample and the pure alpha chromia standard (atca. 33.6 °20 for Cu K radiation) between 32.5 and 35.0°20, subtractingthe background to provide corrected integrated areas and then ratioingthe corrected area for the catalyst sample to the corrected area for thestandard sample.

The catalyst exhibits an X-ray diffraction peak at a spacing of latticeplanes from 2.65 to 2.7 of half maximum peak width less than 0.8degrees.

Preferably the chromium in the catalyst is present as chromium (III)although a small amount, say up to 10%, of chromium (VI) may be presentas a result of the conditions under which the chromia is crystallised.As described hereinafter, crystallinity can be induced in the chromia bysintering the catalyst at elevated temperature and this may be carriedout under an inert atmosphere or in the presence of air. Catalystsproduced by sintering in an inert atmosphere tend to compriseessentially chromium (III) but require higher sintering temperatureswhilst those produced by sintering in air tend to contain some chromium(VI) but require lower sintering temperatures. We prefer to sinter thecatalysts under an atmosphere of air or a mixture of air and nitrogensince these conditions enable relatively low temperatures of 300° C. to450° C. to be employed.

The catalyst of the invention has excellent activity and selectivity andhas improved chemical robustness leading to a long working lifetime.However, the catalyst lacks the physical robustness or toughnessassociated with amorphous chromia catalysts and is difficult to handlein practice, for example it is not readily produced in the form ofpellets in which fluorination catalysts are usually produced and it doesnot easily withstand temperature shocks as are often encountered in theoperation of large-scale industrial plants. This problem can bealleviated by blending the improved partially crystalline catalyst witha non-crystalline chromia so that the catalyst may comprise essentiallyamorphous chromia as well as crystalline chromia. Such blended catalystshave improved toughness and can be pelleted and handled without too muchdifficulty. The amount of the non-crystalline (essentially amorphous)chromia additive may vary within wide limits but will usually be fromabout 10% to 60% by weight of the blended catalyst. The non-crystalline(essentially amorphous) chromia may itself contain a divalent metal, forexample an activity promoting amount of a divalent metal such as zinc,cobalt or nickel.

The partially crystalline catalyst can be produced by sintering thecorresponding amorphous or essentially non-crystalline catalyst orchromium hydroxide precursor thereof at elevated temperature underconditions whereby the apparent degree of crystallinity induced in thechromia is controlled, for example to between 8% and 50% by weight andsuch a process is provided according to another feature of theinvention. Such a process in which the crystallised chromia issubsequently impregnated with zinc or a compound of zinc is alsoprovides a further aspect of the invention.

Sintering may be carried out under an inert atmosphere such as nitrogengas or in an oxidising atmosphere such as air which may optionally bediluted with an inert gas such as nitrogen. The temperature of sinteringmay be within the range from about 400° to 800° C., preferably from 500°C. to 600° C. in an inert atmosphere and from about 300° C. to 800° C.,preferably from 330° C. to 500° C. in air. Catalysts produced bysintering in nitrogen contain the chromium as essentially only chromium(III) whilst those produced by sintering in air tend to contain somechromium (VI) as well as chromium (III). As described hereinbefore, weprefer to sinter the catalyst or precursor thereof in a mixed atmosphereof air and an inert gas such as nitrogen.

The crystallisation of chromia is an exothermic reaction and may beaccompanied by a rapid rise in temperature leading to hot spots orrun-away reaction unless the reaction is controlled. For this reason itis desirable to raise the temperature of the chromia to the desiredsintering temperature and induce crystallisation of the chromia over aperiod of several hours, for example from 1 to 50 hours and preferably 4to 12 hours. We have found that operating in this way enables us tocontrol the reaction and the degree of crystallisation induced in thechromia.

During sintering and crystallisation, the surface area of thechromia/catalyst is reduced generally from above 100 m²/gm to below 100m²/gm, for example from 150 m²/gm to below 70 m²/gm. We have found thatwithin the range of crystallinity 8% to 50%, the surface area of thecatalyst decreases with increasing crystallinity from about 70 m²/gm toabout 20 m²/gm. The surface area of the catalyst at any particular stageof the sintering procedure gives a guide as to the degree ofcrystallinity in the chromia and provides an indication of sufficientsintering. The degree of crystallinity in the catalyst can be controlledby controlling the sintering conditions.

The preferred catalysts containing a divalent metal promotor such aszinc, cobalt or nickel or compounds thereof can be produced by inducingcrystallisation in a chromia catalyst already containing the divalentmetal promotor or by creating the partially crystalline chromia basecatalyst and subsequently impregnating it with the divalent metalpromotor. Any of the known techniques for producing chromia-basedcatalysts can be used to produce the precursor catalyst in whichcrystallinity is induced.

If present, the amount of the divalent metal promotor is known in theart but as discussed hereinbefore in the case of zinc or a zinc compoundthe amount generally should be less than is used in amorphous chromiacatalysts. Further, the optimum amount of zinc promotor to afford anincreased initial catalyst activity depends upon the catalystpreparation method and generally is lower for catalysts made byimpregnation of a pre-crystallised chromia base than for catalysts madeby a route involving coprecipitation of chromium and zinc salts, forexample hydroxides. As a guide, the optimum amount of zinc in a catalystmade by impregnation of a crystalline chrormia may be about 0.5% byweight whilst for a catalyst made by the coprecipitation route theoptimum amount of zinc may be about 1% by weight.

The partially crystalline chromia catalysts of the invention may beblended with conventional amorphous chromia catalysts in order to impartphysical robustness or toughness to the catalyst and enable it to bepelleted and handled without serious damage. As described hereinbefore,the amount of the conventional catalyst additive may be from about 10%to about 60% or even more of the blended catalyst.

The improved catalyst of the invention may be used in any of thefluorinaton reactions in which chromia-based catalysts are normallyemployed. These will usually be reactions of halogenated andparticularly chlorine-containing hydrocarbons with hydrogen fluoride inthe gas phase at elevated temperature. Numerous such reactions areoperated commercially and amongst them may be mentioned the fluorinationof halogenated aliphatic hydrocarbons containing from 1 to 6 carbonatoms, for example methylene chloride (to produce difluoro- methane, HFC32); trichloroethylene (to produce 1,1,1,2-trifluoro-2,2-dichloroethane, HCFC 133a and 1,1,1,2-tetrafluoroethane, HFC 134a);HCFC 133a (to produce HFC 134a); perchloroethylene (to producepentafluoroethane, HFC 125; chlorotetrafluoroethane, HCFC 124; anddichlorotrifluoroethane, HCFC 123); 1,1,2,2-tetrachloroethane (toproduce HFC 134) and dichlorotrifluoroethane (to produce HFC 125). Thecatalyst is also useful in the removal of the impurity chlorodifluoro-ethylene (HCFC 1122) from HFC 134a by reacting the impurity withhydrogen fluoride to produce HCFC 133a. Processes employing the abovestarting materials are used commercially and thus are important but itis to be understood that the fluorination process according to thepresent invention is not limited to use of these starting materials.

Included within the invention is a process for fluorinating halogenatedhydrocarbons which comprises reacting the halogenated hydrocarbon withhydrogen fluoride in the vapour phase at elevated temperature in thepresence of the improved fluorination catalyst described herein. Theconditions such as temperature, pressure, ratios of reactants and numberof reaction steps for carrying out fluorination reactions usingchromia-based catalysts are well known in the art and are generallyapplicable to the improved catalyst of the invention, although theincreased activity of the improved catalyst generally enables lowertemperatures or shorter contact times to be employed than have typicallybeen used hithereto.

When employed in the production of hydrofluorocarbons [HFCs], theimproved catalysts can suffer deactivation due to coke/carbon depositionand may require periodic regeneration. The catalysts can be regeneratedas necessary by conventional regeneration techniques such as heating inair or in a mixed atmosphere of air and hydrogen fluoride and/or aninert gas. The improved catalysts afford the advantage that they requirereplacement less frequently than conventional chromia-based catalystsand have a longer active working lifetime.

The invention is illustrated but in no way limited by the followingexamples.

EXAMPLE 1

An amorphous chromia catalyst containing 1% by weight of zinc wasprepared by the mixed metal hydroxide precipitation technique. 4 litresof 1 molar chromium nitrate [Cr(NO₃)₃] solution were added to 12 ml of 4molar zinc nitrate [Zn(NO₃)₂] solution to form a mixed metal nitratesolution.

740 ml of 0.88 molar ammonia solution was prepared and stirred using animpeller and sufficient of the mixed metal nitrate solution was added toit to lower the pH to 7.3 at a temperature of 21° C. The resulting mixedmetal hydroxide precipitate was collected using a flat bed filter andwashed with demineralised water. The washed precipitate was dried in anitrogen atmosphere for 12 hours at 150° C. and then calcined undernitrogen gas at 280° C. for a further 8 hours. The resulting solid waspowdered, mixed with 2% by weight of graphite and formed into pellets ofdensity 2 gm/cm³. The catalyst at this stage was found to be essentiallyamorphous (non-crystalline) and had a surface area of 239 m²/gmdetermined by the BET nitrogen absorption method.

The catalyst pellets were crushed and seived to generate granules ofparticle size 0.5-1.5 mm and 4 g of the granules was charged to a 9 mminternal diameter reaction tube for sintering. The catalyst was heatedat 425° C. for 16 hours in a flow of 18 m/min of nitrogen mixed with 1ml/min of air after which time the air flow was stopped and the catalystwas cooled to room temperature in the nitrogen flow. The catalyst wasthen discharged from the reactor and was found to have an apparentcrystallinity of about 45% with a surface area of 57 m²/gm measured bythe BET nitrogen absorption method.

2 gm of the partially crystalline catalyst was re-charged to the reactorfor conditioning and activity testing. The catalyst was dried at 300° C.for 30 minutes in a nitrogen flow of 50 ml/min and then was heated at300° C. in a hydrogen fluoride flow of 20 ml/min until hydrogen fluoridewas detected in the reactor vent stream. The reactor temperature wasincreased to 380° C. for 16 hours whilst continuing the flow of hydrogenfluoride, prior to measurement of the activity of the catalyst.

The catalyst was cooled to 350° C., still in the flow of hydrogenfluoride, and then 5 ml/min of chloro-2,2,2-trifluoroethane [HCFC 133a]was added to the hydrogen fluoride flow to generate a feed having an HF:HCFC 133a molar ratio of 4:1. After 2 hours, the catalyst temperaturewas reduced to 300° C. and the yield of 1,1,1,2-tetrafluoroethane [HFC134a] at 300° C. was quantified by gas chromatographic analysis. Theyield of HFC 134a at 300° C. was 17.2%

COMPARATIVE EXAMPLE A

For purposes of comparison, the activity of the unsintered catalyst wasdetermined. 2 gm of the amorphous catalyst granules was charged into thereactor and the catalyst was dried, conditioned and tested by theprocedure described above except that the sintering step at 425° C. wasomitted so that the catalyst remained essentially non-crystalline. Theyield of HFC 134a at 300° C. was 7.6%.

COMPARATIVE EXAMPLE B

For purposes of comparison also, an amorphous chromia catalystcontaining 3% by weight of zinc was prepared as described in Example 1using 36 ml of the zinc nitrate solution instead of 12 ml. The resultingcatalyst had a surface area of 183 m²/gm. The catalyst was granulatedand sieved as in Example 1 and 4 gm of catalyst granules was charged tothe reactor for sintering. The catalyst was heated at 400° C. for 16hours in a flow of 5 ml/mn of air after which time the catalyst wascooled to room temperature in a nitrogen flow of 18 ml/min. The catalystwas discharged from the reactor and was found to have an apparentcrystallinity of about 90% with a surface area of 23 m²/gm. Theamorphous and crystalline catalysts were tested as described above.Using the amorphous catalyst, the yield of HFC 134a at 300° C. was 8.6%and using the crystalline catalyst, the yield of HFC 134a at 300° C. wasonly 1.8%.

EXAMPLE 2

An amorphous chromia catalyst was prepared by the precipitationtechnique. Aqueous ammonia solution was added to an aqueous solutioncontaining chromium to produce a precipitate of chromium hydroxide. Theprecipitate was washed with demineralised water, dried in a nitrogenatmosphere at 150° C. and then calcined under nitrogen at 280° C. for 8hours. The resulting solid was powdered, mixed with 2% by weight ofgraphite and formed into pellets. The chromia was found to beessentially amorphous (non-crystalline) and had a surface area of 176m²/gm determined by the BET nitrogen adsorption method.

The amorphous catalyst pellets were crushed and seived to generategranules of particle size 0.5-1.4 mm and 50 gm of the granules wascharged to a reaction tube for sintering. The catalyst was heated at190° C. in a flow of 20 ml/min nitrogen gas for 2 hours and then thetemperature was raised to 550° C. at the rate of 20° C./hour andmaintained at 550° C. for 24 hours. The catalyst was then cooled to roomtemperature in the nitrogen flow and discharged from the reactor. Thisbase catalyst was found to have an apparent degree of crystallinity ofabout 80% with a surface area of 47 m²/gm. 4.95 gm of the base catalystwas added to 0.96 ml of aqueous zinc chloride solution (prepared bydissolving 13.54 gm of zinc chloride in demineralised water to provide250 ml of solution) and the mixture was stirred and evaporated todryness to give an impregnated chromia catalyst containing 0.5% byweight of zinc.

2 gm of the impregnated catalyst was charged to an Inconel reaction tubefor conditioning and activity testing. The catalyst was dried at 250° C.for 90 minutes in a 50 ml/min flow of nitrogen gas and was then heatedat 300° C. in a 20 ml/min flow of hydrogen fluoride until hydrogenfluoride was detected in the reactor vent stream whereupon thetemperature was raised to 380° C. for 16 hours whilst the flow ofhydrogen fluoride was maintained.

After conditioning as above, the catalyst was cooled to 350° C., stillin the hydrogen fluoride flow and then 5.8 ml/min of1-chloro-2,2,2-trifluoroethane [HCFC 133a] was added to the hydrogenfluoride flow to provide a feed having an HF:HCFC 133a molar ratio of3.4:1. After two hours the catalyst temperature was reduced to about orbelow 300° C. The yield of 1,2,2,2-tetrafluoroethane [HFC 134a] at 297°C. and 288° C. was measured by gas chromatographic analysis. The yieldof HFC 134a at 297° C was 17.4% and the yield at 288° C. was 14.1%.

EXAMPLE 3

Using the impregnation procedure described in Example 2, an impregnatedchromia catalyst containing 1% by weight of zinc was prepared from 4.90gm of base catalyst and 1.92 ml of zinc chloride solution. 2 gm of thecatalyst was conditioned and tested as described in Example 2 with ayield of HFC 134a at 297° C. of 14% and a yield of HFC 134a at 288° C.of 11.5%.

EXAMPLE 4

Using the impregnation procedure described in Example 2, an impregnatedchromia catalyst containing 3% by weight of zinc was produced from 4.69gm of base catalyst and 5.77 ml of zinc chloride solution. 2 gm of thecatalyst was conditioned and tested as described in Example 2 with ayield of HFC 134a at 303° C. of 7.4% and a yield at 292° C. of 6.1%.

COMPARATIVE EXAMPLE C

For purposes of comparison the activity of the base chromia catalyst(not impregnated with zinc) was determined using the conditioning andtesting procedure described in Example 2. The yield of HFC 134a at 301°C. was 15% and at 283° C. was 6.4%.

What is claimed is:
 1. A chromia-based fluorination catalyst comprisingzinc or a compound of zinc in an amount of less than about 3% by weightof the catalyst, and having an apparent degree of crystallinity of from8% to 50% by weight, as determined by X-ray diffraction analysis usingthe standard NIST technique and comparing the result obtained with thatobtained by analysis of pure alpha chromia standard prepared bysintering chromia at 1223K in air for 24 hours, which catalyst isobtained by inducing crystallinity in chromia and subsequentlyintroducing zinc or a compound of zinc into the crystallized chromia byimpregnation with a solution of soluble zinc salt or by inducingcrystallization in a chromia catalyst comprising zinc or a zinccompound, essentially in the absence of H₂ gas.
 2. A catalyst as claimedin claim 1 in which the zinc or compound of zinc is present in an amountexceeding 0.1% by weight of the catalyst.
 3. A catalyst as claimed inclaim 1 in which the zinc or compound of zinc is present in an amount of0.1 to 2% by weight of the catalyst.
 4. A catalyst as claimed in claim 1having a surface area greater than about 20 m²/gm.
 5. A chromium-basedfluorination catalyst composition comprising a blend of a catalyst asclaimed in claim 4 with a non-crystalline chromia catalyst.
 6. Acatalyst composition as claimed in claim 5 in which the amount of thenon-crystalline catalyst component of the blend is from about 10 toabout 60% by weight of the blended catalyst composition.
 7. A catalystas claimed in claim 1 in which the chromia exhibits an apparent degreeof crystallinity of greater than 20% by weight.
 8. A process forproducing a fluorinated hydrocarbon, which comprises reacting ahalogenated hydrocarbon with hydrogen fluoride in the vapour phase atelevated temperature in the presence of a catalyst as claimed inclaim
 1. 9. A process as claimed in claim 8 for producing 1,1,1,2tetrafluoroethane by reacting 1,1,1-trifluoro-2-chloroethane withhydrogen fluoride.
 10. A process for producing a chromia-basedfluorination catalyst comprising zinc or a compound of zinc in an amountof less than about 3% by weight of the catalyst, which process includesthe step of sintering an essentially non-crystalline chromia catalyst orprecursor thereof at a temperature of from 400 to 800° C. until it hasan apparent degree of crystallinity of from 8% to 50% by weight, asdetermined by X-ray diffraction analysis using the standard NISTtechnique and comparing the result obtained with that obtained byanalysis of a pure alpha chromia standard prepared by sintering chromiaat 1223K in air for 24 hours.
 11. A process as claimed in claim 10 whichcrystallinity is induced in the chromia by sintering and in which zincor compound of zinc is subsequently introduced into the crystallisedchromia by impregnation with a soluble zinc salt.
 12. A process asclaimed in claim 11 in which the crystallinity is induced to the extentthat the chromia exhibits an apparent degree of crystallinity of greaterthan 20% by weight.
 13. A process as claimed in claim 11 in which theamount of zinc or zinc compound introduced is such that the zinc or acompound of zinc comprises an amount exceeding 0.1% by weight of thecatalyst.
 14. A process as claimed in claim 11 in which the amount ofzinc or zinc compound introduced is such that the zinc or a compound ofzinc comprises 0.1 to 2% by weight of the catalyst.
 15. A process asclaimed in claim 11 in which the degree of crystallinity in the chromiais controlled so as to result in a catalyst having a surface areagreater than about 20 m²/gm.
 16. A chromium-based fluorination catalystcomprising from 0.1 but less than about 3% by weight of zinc or acompound of zinc wherein the chromia is at least partially crystallineand exhibits an apparent degree of crystallinity as represented by alphachromia crystals of greater than 8% and less than 50% by weight andwherein the catalyst has a surface area greater than about 20 m²/gm. 17.A catalyst as claimed in claim 16 in which the zinc is present in anamount of up to 2% by weight.
 18. A catalyst as claimed in claim 16having a surface area in the range of from about 30 to about 70 m²/gm.19. A catalyst as claimed in claim 5, wherein the non-crystallinechromia catalyst contains an activity-promoting amount of divalent metalselected form zinc, cobalt, nickel and magnesium.
 20. A process forproducing a fluorinated hydrocarbon, which comprises reacting ahalogenated hydrocarbon with hydrogen fluoride in the vapour phase atelevated temperature in the presence of a catalyst as claimed in claim5.
 21. A process as claimed in claim 20 for producing 1,1,1,2tetrafluoroethane by reacting 1,1,1-trifluoro-2-chloroethane withhydrogen fluoride.